The present invention relates to a dual polarization transmission system which transmits digital modulated signals having different bandwidths by radio waves that have the same center frequency and are different in polarization and, more particularly, to the cancellation of cross polarization interference components.
A transmission system of the type described uses a co-channel frequency arrangement in which two radio channels of horizontal polarization and vertical polarization share the same frequencies, and a particular bandwidth is assigned to each polarization. The co-channel frequency arrangement promotes effective use of frequencies without increasing the interference between co-polar channels, compared to an interleave frequency arrangement in which radio channels are alternately arranged on a horizontally and a vertically polarized wave. This type of transmission system is disclosed in U.S. Pat. No. 4,811,021 issued to Yoshimoto and Maeda, Maeda being one of the inventors of the present invention.
A system of the type transmitting signals having the same bandwidth by use of the co-channel frequency arrangement of radio channels on a horizontally (H-) and a vertically (V-) polarized wave is also known in the art. In relation to this type of system, there has been proposed an implementation for cancelling cross polarization interference, i.e., the interference between a horizontally and a vertically polarized wave by Ryu, Tahara and Noguchi in the paper entitled "IF BAND CROSS POLARIZATION CANCELER" reported in ICC '84 LINKS FOR THE FUTURE, IEEE International Conference on Communication, heldin Amsterdam, The Netherlands, May 14-17, 1984. (IEEE, 1984). It is not practicable, however, to apply the cross polarization interference cancellation (XPIC) for use with the co-channel frequency arrangement in which a horizontally and a vertically polarized wave have the same bandwidth directly to the co-channel type transmission system which transmits signals having different bandwidths described above.
Specifically, an interference canceller for cancelling the interference between a H- and a V- polarized wave has a transversal filter therein. In the co-channel arrangement wherein the radio channels share the same bandwidth, the tap delay of the transversal filter is selected to be the reciprocal of the symbol rate f.sub.s (equal to bandwidth) of signals to be transmitted. XPIC using such a transversal filter is not directly applicable to the dual polarization transmission system disclosed in the '021 patent, since the latter assigns a different symbol rate to each of a H- and a V- polarized wave.